1. Field of the Invention
The present invention relates to a three-dimensional photonic crystal having a three-dimensional periodic refractive-index structure, and a manufacturing method thereof.
2. Description of the Related Art
Yablonovitch, Physical Review Letters, Vol. 58, p. 2059, 1987 proposes a concept of controlling the electromagnetic wave's characteristic, such as a transmission and a reflection, through periodically arranged structures as large as or smaller than a wavelength. This structure is known as a photonic crystal, and implements an optical element having a reflectance of 100% without a loss of the light in a predetermined wavelength region.
The operation that makes the reflectance 100% in the predetermined wavelength region is referred to as a photonic band gap (operation), in comparison with the energy gap of the semiconductor.
Furthermore, A three-dimensional fine periodic structure provides a photonic band gap to the incident light from all directions. This is also referred to as a complete photonic band gap (operation).
With the complete photonic band gap, a variety of applications including a control of a spontaneous emission in a light-emitting element are available, and a new functional device that does not conventionally exist can be implemented. Therefore, a functional device configured to implement the complete photonic band gap in a wider wavelength region.
Conventionally, U.S. Pat. Nos. 5,335,240, 5,440,421, and 6,597,851 propose some structures each having a complete photonic band gap operation.
U.S. Pat. No. 5,335,240 discloses a woodpile structure that has a stack of a plurality of layers, each of which arranges a plurality of columnar structures in parallel having a constant sectional shape. In the plural layers, directions of columnar structures alternately differ by 90°.
U.S. Pat. No. 5,440,421 discloses a structure that arranges and partially overlaps a plurality of parallel columnar structures in a stacking direction, and has a plurality of holes that extend in the stacking direction.
U.S. Pat. No. 6,597,851 discloses a stacked structure that arranges a plurality of layers while shifting them by a length that is ⅓ as long as a fundamental period, each layer having a plurality of hexagonal columnar structures that extend in the stacking direction on a flat part that has a plurality of holes.
The woodpile structure proposed in U.S. Pat. No. 5,335,240 has an advantage of a simple structure and an easy manufacture because four layers constitute one period.
Since the photonic band gap of the woodpile is narrower than that of the inverse opal, it is difficult to realize a device (such as a waveguide and a wavelength selective filter) which can operate in a wide operational band. However, the inverse opal that can provide the device that can operate in the wide operational band is hard to manufacture by the conventional semiconductor process.
In addition, the structure proposed in U.S. Pat. No. 5,440,421 also has the complete photonic band gap, but requires a plurality of very deep holes, and is hard to manufacture.
Moreover, the structure proposed in U.S. Pat. No. 6,597,851 has smaller anisotropy and a comparatively wider photonic band gap width than the woodpile structure. However, it is difficult to manufacture because six layers constitute one period, and require an extremely high alignment precision among these layers.